A self-excited reactive power compensation apparatus called “STATCOM (Static Synchronous Compensator),” “SVG (Static Var Generator),” “self-excited SVC (Static Var Compensator)” or the like is employed in many cases to enhance stability of a system by controlling system reactive power. The self-excited reactive power compensation apparatus is effective not only at enhancing stability of the system during steady operation but also at transiently enhancing stability of the system during system trouble or after the trouble is removed.
In order to achieve the above purpose, a control circuit of the self-excited reactive power compensation apparatus is generally configured as follows: the control circuit includes a voltage control loop (main loop) that outputs a reactive current command such that a system voltage follows a desired system voltage command, and a current control loop (sub loop) that controls an output voltage of a power converter such that an output current of the power converter follows this reactive current command.
However, when the system reactive power is controlled only by feedback control based on actual voltage and current, a desired response speed cannot be obtained in many cases because of a delay of a feedback control system. In the case where the self-excited reactive power compensation apparatus cannot follow sudden fluctuations in the system voltage that occur at the time of the system trouble due to a delay in response, an overcurrent may flow through the converter. In this case, it can be considered that the self-excited reactive power compensation apparatus stops to protect itself.
Japanese Patent Laying-Open No. 6-233544 (Patent Document 1), for example, discloses a semiconductor power conversion apparatus capable of controlling an output alternating current at high speed in accordance with a set alternating current. This power conversion apparatus includes a feedforward power control circuit that generates an output voltage command of a semiconductor power converter based on the phase and the amplitude of the set alternating current. The output voltage command from the feedforward power control circuit is corrected based on a difference between the set alternating current and a system current. Furthermore, the power converter is controlled based on a sum of a system voltage and the corrected output voltage command.    Patent Document 1: Japanese Patent Laying-Open No. 6-233544